The invention relates to a method and apparatus for preheating raw materials for glass production using waste heat from the glass melting process. The invention uses electrostatic forces to improve the performance of conventional methods of preheating glass batch materials. Additionally, the electrostatic forces act to remove fine particulate matter from the glass furnace exhaust gases, thus achieving simultaneous pollution emission reduction.
Glass is made by heating and melting a mixture of solid raw materials to a liquid state. The melting is done inside of a furnace and necessarily requires substantial amounts of heat. Typically, this heat is generated by the combustion of fossil fuels with the exhaust gases from the combustion leaving the furnace. Exhaust gas temperatures immediately after the furnace are quite high, typically 1300-1450xc2x0 C. Combustion air preheaters are normally included which recover some of the heat in these gases. Even so, gas temperatures at the discharge to atmosphere are quite high, thus substantial amounts of heat are wasted. The cost of fuel for the furnace is a major component in the cost of making glass.
The raw materials for glass are typically called batch. The word frit generally refers to an assemblage of various pulverous materials including silica sand, limestone, soda ash, salt cake, and a variety of other minor ingredients. The material and mixture ratios are carefully chosen to produce glass of the desired properties and quality. Generally these materials are prepared in a finely divided form to promote their melting rates. Sizes are typically 100 to 200 xcexcm diameter with a maximum size of 1 mm. Also, recycled glass, either from the factory or from external sources, is used as a raw material. This recycled glass is called cullet. For ease of handling cutlet is generally crushed to sizes less than 50 mm before use in the glass furnace. Sometimes, cullet is pulverised to sizes similar to the other raw materials. For the purpose of further description, the word batch will be used to mean frit alone, cullet alone, or a mixture of frit and cullet, the cutlet being either crushed or pulverised.
Glass furnaces emit various pollutants with their exhaust gases, most commonly particulate matter (fine dust) and SO2. The particulate matter is especially difficult to collect because it is of diameter less that 1 xcexcm and the exhaust gas temperatures are high. Typically very large electrostatic precipitators are utilised, often preceded by absorption towers for chemical reaction of various reagents with the SO2pollution. Other gaseous pollutants are sometimes present, specifically, HF and HCl acid gases. Many local regulations require reduction of the amounts of these pollutants that are emitted to the atmosphere.
DESCRIPTION OF PRIOR ART
The most relevant prior art is xe2x80x9cMethod and Device for Preheating Raw Materials for Glass Production, Particularly A Cullet Mixturexe2x80x9d, U.S. Pat. No. 4,696,690. A brief description of this device is beneficial to an understanding of the present invention. In this device, furnace exhaust gases are passed through xe2x80x9cflow ductsxe2x80x9d inside of a batch bunker. The upper half of the flow ducts are formed by equal sided angle sections arranged to form a roof like structure inside the bunker. The angles provide an open bottom and the batch itself forms the bottom half of the flow duct, due to its angle of repose under the roof.
Batch is introduced to the bunker through its open top. The batch is moved downward by gravity, thus providing continuously renewed surfaces in the flow ducts that are exposed to the furnace exhaust gases. Heat is transferred to the batch primarily because of the direct contact with the hot gases.
The flow ducts are arranged in horizontal banks, with the furnace gas divided to flow through the ducts of a given bank in parallel. Multiple banks of flow ducts are provided one above another and internal tunnels are provided to direct the flow successively from the lower banks to the upper banks. The result is to achieve a countercurrent flow of hot gases with the downward moving batch in the bunker. Batch is fed out of the device through a nozzle as controlled by a conventional device such as a vibratory, screw, or other mechanical type feeder.
Such devices have been successfully and reliably operated, but have found limited applicability in the glass industry. Preheating of batch and reduced fuel consumption of the furnace have been well demonstrated. As well, partial removal of SO2, HCl, and HF has been realised. The fundamental limitation of the device is that dust from the batch material is entrained in the furnace gases flowing through the flow ducts. This entrainment can be minimised by reducing the gas flow velocity in the ducts, but such an approach greatly increases the size and cost of the device. Typically, electrostatic precipitators have been installed downstream of the device, in order to capture the entrained dust and prevent its release to atmosphere. However, such an amalgamation results in an expensive overall installation. The economic benefits of preheating the batch materials cannot justify the full installation cost of the equipment.
What is needed is an improvement to the device to allow it to operate without entrainment of dust. Furthermore, in cases where the glass manufacturing process is faced with governmental legislation to reduce pollutant emissions from the furnace, a device capable of simultaneous particulate matter and SO2 removal from the furnace exhaust gases to meet these regulations would eliminate the need for additional pollution control equipment. Then, the process of this invention would result in an overwhelming economic justification for installation. This is the motivation and result of the invention described here.
In view of the foregoing, the following are objects or benefits of the described embodiment of the present invention:
1. Preheat batch material for use in a glass-melting furnace using heat from the furnace exhaust gases by direct contact with the gases.
2. Eliminate carryover of dust with the gases leaving the direct contact process.
3. Simultaneously remove fine particulate matter from the incoming gases, thus achieving a reduction of a pollution emission.
4. Simultaneously remove gaseous pollutants, which might be components of the incoming gas, from the incoming gases by chemical reaction with a constituent of the batch and formation of a solid reaction product.
5. Optimise efficiency of the process by operating with high gas velocity and countercurrent flow of gas and solids.
One aspect of the present invention is directed to a method or process for achieving the above-described objects that can be described as including the following steps:
a) providing a bulk quantity of glass batch material in a hopper;
b) providing at least one gas flow tunnel through the bulk quantity of batch material, said tunnel is of cylindrical shape and is generally horizontally disposed within the hopper;
c) the top portion of the cylindrical tunnel consists of electrically conductive plates forming roofs within the bulk material and in contact with said batch material;
d) said roofs are arranged so that the bottom portion of the cylindrical tunnel consists of batch material residing at its gravitational angle of repose beneath the roofs;
e) passing glass furnace exhaust gases through said tunnel;
f) providing an electrically conductive electrode within said tunnel;
g) applying an electrical potential difference between said electrode of step f) and said roofs of step c) of sufficient magnitude to produce corona discharge inside the tunnel, said corona discharge consisting of a flow of gaseous ions through the furnace gases, at least a portion of said gaseous ions flowing into the glass batch material;
h) supplying unheated batch material to the top of said hopper; and
i) removing heated batch material from the bottom of said hopper so that the batch material moves through the hopper by action of gravity.
Another aspect of the present invention is directed to an apparatus for carrying out the foregoing process.
In another embodiment of the invention, fine dust pollutants in the gases are removed from the gases by electrostatic precipitation onto the inside of said flow tunnels, including precipitation directly onto the batch surface, and are subsequently removed from the hopper along with the batch material.
In still another embodiment of the invention, gaseous pollutants introduced with the gases are chemically reacted with a constituent of the batch material to form a solid reaction product that is removed from the hopper with the batch material.
In still another embodiment of the invention, a plurality of such tunnels is provided, the gases are first introduced to the tunnels near the bottom of the hopper and are finally removed from tunnels near the top of the hopper. Gases are directed to flow successively through the tunnels, or banks of tunnels, from the bottom to the top of the hopper. The batch is introduced at the top and removed from the bottom of the hopper so that the gas and batch flow in direction generally countercurrent to each other.
The invention can be advantageously used to preheat the batch using heat from the exhaust gases. By preheating these materials before they are introduced to the furnace, the amount of fuel required for heating and melting them in the furnace can be reduced. This fuel reduction can represent a substantial economic benefit to the glass making process and also reduces the emission of the so-called green house gases (such as NOx and CO2) simply because less fuel is burned. Simultaneous with this preheating, pollutant emissions such as fine dust, SO2, HCl, and HF can be reduced with high efficiency to satisfy stringent regulations. SO2, HCl, and HF emissions can be reduced because most glass batch contains substantial amounts of material which is chemically reactive with these acid gases, specifically soda ash (Na2CO3) and limestone (CaCO3).
Fossil fuel fired glass furnaces are of several different designs. When air is combusted with fuel, the air is typically preheated in regenerative or recuperative heat exchangers, utilising some of the waste heat exiting the furnace. As well, nominally pure oxygen can also be used for combustion, in which case no waste heat recovery equipment is typically involved.
While the invention could be advantageously applied to any of the glass production schemes, its benefits are greatest in the case of oxygen-fuel fired furnaces. This is because exhaust gas temperatures are higher, thus batch can be preheated to high temperatures, and because reduction in fuel requirements for the furnace is accompanied by a proportional reduction in the oxygen supply (and thus cost) for the furnace.